Carbon Source Utilization Profiles for Microbial Communities from Hydrologically Distinct Zones in a Basalt Aquifer

Abstract

The Eastern Snake River Plain aquifer has hydrologically distinct zones in basalt flow units and interbedded sediments. The zones that differ markedly in physical features (e.g., porosity and permeability) have similar groundwater chemistries. The primary objective of this study was to determine whether intervals within the aquifer that contrast on the basis of permeability have distinct communities of unattached microorganisms based on functional attributes. Aquifer sampling was conducted using a submersible pump to obtain whole-well (w) samples, and a straddle-packer pump (SPP) to obtain samples from specific aquifer intervals that were vertically distributed in the open borehole. The SPP intervals ranged from 4.6 to 6.1 m in length and were located from 142 to 198 m below land surface. A community-level physiological profile (CLPP) was used to determine functional characteristics of the microbial community in the groundwater samples based on the community response to 95 sole organic carbon sources. Surface soil samples at the site were analyzed in a similar manner for comparison. The total bacterial population in the groundwater samples was determined using acridine orange direct counts. Principal components analysis (PCA) of the CLPP dataset distinguished between surface soil and aquifer microbial communities. Soils scored low in the respiration of polymers, esters, and amines and high in bromosuccinate, when compared to aquifer samples. The W samples were distinct from SPP samples. The 180- to 198-m interval, with the lowest hydraulic conductivity of all intervals, yielded samples that grouped together by PCA and cluster analysis. Direct counts varied between 10⁴ and 10⁵ cells ml⁻¹, and showed no relationship to the depth of the sample or to the hydraulic conductivity of the sample interval. Differences between microbial communities based on respired carbon compounds were discerned in separate, hydrologically distinct intervals within the borehole, although these differences were slight. Differences among aquifer intervals were less apparent than differences between surface soils and groundwater, and may be related to variations in hydrologic properties over the intervals sampled. The results suggest that free-living microbial communities in basalt aquifers, as characterized by CLPP are relatively unaffected by wide ranges in hydraulic conductivity when other abiotic factors are essentially equal.